1. Field of the Invention
An object of the present invention is a method for the imaging of intra-voxel movements by nuclear magnetic resonance (NMR) in a body. It can be applied more particularly in medicine where NMR imaging is an indispensable diagnostic tool.
2. Description of the Prior Art
A known method of NMR imaging is described in the European patent No. 86 401 423.8. In this method, it is explained that the depiction of standard images by highlighting the spin-lattice relaxation times T.sub.1 and the spin-spin relaxation times T.sub.2 of the magnetic moments of the particles of a body being examined is not always enough to differentiate certain healthy tissues from neighboring tumors and to identify lesions. For improved differentiation, it was proposed to measure and depict the distribution of the molecular diffusion coefficients of the imaged tissues. In the development and perfecting of this invention, it emerged that the method used to reveal the molecular diffusion coefficients did not circumvent the effect of the more comprehensive molecular movements within the volume elements taken into account, corresponding to the necessary discretization of an image to enable the computation of this image. These volume elements are called voxels. For, group movements such as movements of blood molecules create micro-circulation motions which become identified with the molecular diffusion characteristics of the tissues examined.
In the above-mentioned patent application, it was shown that, by repeating an imaging experiment with different experimental characteristics and by comparing the results obtained at the end of the first experiment with those obtained at the end of the second experiment, it was possible to depict two images. With these two images, it was possible to present the molecular diffusion coefficients independently of the coefficients of blood perfusion of the tissues and vice versa.
In practice, the first experiment comprises two series of radiofrequency excitations of the body under NMR examination: a first series known as a series with little diffusion and a second series known as a diffusive series. The second experiment has only a third series of radiofrequency excitations, also called diffusive but having characteristics of sensitization to molecular diffusion which are different from the characteristics of sensitization to the molecular diffusion of the second series of excitations. For, in the second experiment, it is unnecessary to reiterate the series of excitations with little diffusion. A series of radiofrequency excitations more precisely comprises a series of excitation/measurement sequences. In each sequence, the body to be examined is made to undergo an excitation, at the end of which the NMR signal resulting from this excitation is measured. During the excitation, magnetic encodings of space are additionally applied to the body so as to cause encodings of the measured signal in order to extract therefrom, by decoding, information directly representing luminosity values to be assigned to the different pixels of the image. These encoding/decoding operations constitute imaging methods. These encodings are applied in the form of magnetic field gradient pulses. A preferred imaging method described in the above-mentioned patent application is the 2DFT method. The essential feature of the invention which wa the object of the above-mentioned patent application resides in the fact that there is a common duration to all the sequences of every series of experimental sequences.
In the excitation-measuring sequences, the NMR signal resulting from the excitation is known to fade away very quickly after excitation, namely before the additional magnetic encodings can be efficiently applied to the regions to be imaged, prior to the measurement. This fading away is due to non-homogeneity in the orienting magnetic field in the body examined. This non-homogeneity causes a corresponding dispersal in the phase of the NMR signals emitted by the different particles distributed in space. It has become customary to cause this phase dispersal to be reflected by applying an additional excitation pulse, called a spin-echo pulse or, again, a 180.degree. pulse because it causes the orientation of the magnetic moments of the particles to be imaged to flip by 180.degree.. At the end of a period which is twice the interval between the initial excitation and the application of this 180.degree. excitation pulse, the signal is revived and can be measured. Typically, each excitation-measurement sequence of this type is separated from the following one by 400 to 1000 milliseconds (repetition time).
The greater the number of excitation-measurement sequences of each series of excitations, the more precise is the definition of the images presented. Ultimately, the images determined at the end of each series are all the more precise if the time taken to acquire them is long. Since it is necessary to acquire three series of excitations, the total period during which a patient is subjected to an NMR excitation of this type may rule out this kind of examination. This approach entails examination periods of about 20 minutes. In general, to resolve problems related to the duration of the experiment, the excitation method currently used is the so-called SSFP (steady state free precession) method. In this method, the excitations are very close to one another in time and cause the magnetic moments of the nuclear particles of the body to flip each time by a relatively small angle depending on the period between the excitations.
It has been shown that this method, although it has no 180.degree. spin-echo excitation pulse, causes the revival of composite NMR signals formed by echos of free precession signals. These revived signals can be used as measurable NMR signals. It has thus been shown that a state of dynamic equilibrium can be created between a longitudinal magnetization M.sub.z of the magnetic moments of the particles and the transversal magnetization M.sub.xy of these magnetic moments by choosing a radiofrequency pulse corresponding to a flip-over angle known as the angle of Ernst, the value of which depends on the repetitivity of the excitation pulses of the sequence and on the average spin-lattice relaxation time of the particles sought to be represented.
When no gradient pulse is applied, the phenomenon occurs simply. On the contrary, for imaging, space encoding pulses must be applied during each sequence in order to differentiate between the contributions of the different particles in the NMR signal. It has been shown that the dynamic equilibrium of the transversal magnetization M.sub.xy is got by re-phasing the transversal magnetization of the moment of each pulse. The re-phasing is got by reversing the direction of the gradient at the origin of the phase shift: by compensating, at the end of each sequence, for the phase shift effects due to the encoding gradients of the image. The reappearance of the NMR signal due to this re-phasing is called the "gradient echo". This notion of phase-shifting/rephasing concerns only the immobile particles placed at different positions in the magnetic field of the machine.
After a number of excitation pulses have been applied, it may be assumed that dynamic equilibrium has been set up. In this mode, a fall is observed in the NMR signal, after each pulse, as well as a rise in this signal, before each pulse. The fall (evanescent) may be considered to be the equivalent of a free precession signal of a standard sequence. The rise (echo) may also be considered to be the equivalent of a spin-echo signal. The NMR signal can be measured in advance of the measurement of this rising signal in the sequence. This can be got by applying additional gradient pulses which destroy the component of the NMR signal, related to the free precession signal, by phase-shifting. The effect of these additional gradient pulses, therefore, is firstly, to separate the reading signals according to their origin: namely, those coming from the free precession signal and those coming from the rising signal. They also have the effect, secondly, of advancing, in the excitation-reading sequence, the period during which the reading is done so that this reading of the NMR signal does not take place precisely when the excitation is applied.
In the invention, advantage has been taken of the existence of fast methods of the SSFP type to produce apparent molecular diffusion images or, by once again reiterating the series of excitations, true molecular diffusion and micro-circulation images. In principle, SSFP type methods are not indicated for these images because the sensitizing of the NMR signal to the molecular diffusion effect requires the application of strong diffusing gradient pulses, i.e. pulses with high amplitudes and long periods. Now, in an SSFP sequence, the period for which these pulses are applied is necessarily short since the sequence is itself short. The principle of the invention is based, nonetheless, on the reiteration of an SSFP sequence, with a diffusing gradient pulse force designed to remove the free precession component and to sensitize the sequence to diffusion and micro-circulation movements, which is different, in a second series of sequences, from a first series. By contrast, for the different series of sequences, the same excitation-reading characteristics are retained (in particular, the same repetition time) as also the same imaging characteristics (preferably, the depiction of comparable images is sought, with one and the same definition, hence with one and the same number of sequences in the series of sequences). It has been shown that there is an integration effect of these pulses of this diffusing gradient which then leads to a paradoxically high level of sensitivity.